Become a moneylender.
If you already have some extra cash on hand, you can earn even more by lending it out and charging interest on your loans. There are many companies, the largest of which are Prosper and Lending Club, that match prospective lenders with prospective borrowers. Although the industry has been shifting away from private investors, there are still opportunities to be had.
If you wish to become a money lender, be sure to abide by any and all applicable laws in your area.